The present invention relates to refractory metal nitride powders particularly those of Ta, Nb and more particularly to such powder as usable in solid electrolyte capacitors.
Solid tantalum capacitors are built on tantalum metal substrates with a dielectric layer composed of anodic tantalum oxide. A well-known problem with this structure is instability caused by oxygen migration from the oxide to the metal substrate during thermal cycling (ref. 1). A gradient of oxygen deficiency with an associated conductivity profile is created across the anodic oxide film.
The present invention focuses on capacitors using a porous mass of sintered nitrides (particularly TaN, Ta2N, NbN) as the anode and nitride powders suitable for making them. The powders are referred to as “substrates” in relation to thin conversion “coatings” of dielectric oxide formed at the surfaces of the sintered powders of an anode slug (of various geometric forms) when contacted by a wet electrolyte under electro-formation conditions. In the final product the pores of the slug can be filled with a wet or solid electrolyte, but the invention is specially beneficially for solid electrolyte capacitors. Capacitors containing a conductivity profile have high bias, frequency and temperature dependence of capacitance. Although capacitor manufacturers have developed procedures to minimize or eliminate the oxygen deficiency and associated conductivity profile in the dielectric, the thermodynamic instability at the metal-metal oxide interface can contribute to ignition and other failures of solid electrolyte tantalum capacitors.
Work over the last 30 years (ref. 2 and references therein) has demonstrated that anodic oxide films grown on tantalum substrates doped with nitrogen are more stable to thermal cycling. If the nitrogen content in the tantalum exceeds 10 at, %; oxygen migration across the tantalum-tantalum oxide interface is suppressed. Capacitors made using a nitrogen doped tantalum substrate are significantly less sensitive to the effects of thermal cycling. In most cases, the substrates were thin films produced by sputtering tantalum in a nitrogen atmosphere. Sputtered substrate capacitors are found as micro devices in integrated circuits. The sputtered capacitors made using a Ta2N (33 at. % N) substrate were especially stable.